I tried again by centering the pivot between the agate covers (y axis, looking down) thinking that perhaps it was binding on one of the covers. This worked a few times, but again inconsistency reared its ugly head. I was able weigh the included 250gr weight perfectly and allow it zero on its own twice, and then began getting readings of 250.4, 250.5, and failing to zero.

The knife edges are a perfect machined edge. The paint was stripped where it was machined - is this the case on yours as well? Perhaps a "refurbished" unit?